In line with miniaturization of portable electronic devices, high integration, and development of hybrid electric vehicles (HEVs) and electric vehicles (EVs), there is demand for secondary batteries having high energy density.
Lithium-sulfur batteries are battery systems using sulfur as a cathode active material and lithium metal as an anode active material. During discharging, sulfur at a cathode is reduced by receiving electrons and lithium at an anode is oxidized through ionization.
Sulfur reduction is a process whereby a sulfur-sulfur (S—S) bond is converted into a sulfur anion by receiving two electrons. Meanwhile, lithium oxidation is a process whereby lithium metal is converted into lithium ions by releasing electrons and the lithium ions are transferred to a cathode through an electrolyte to form sulfur anions and a salt.
Sulfur prior to discharging has a cyclic S8 structure and is converted into lithium polysulfide by reduction reaction. Lithium polysulfide is completely reduced to form lithium sulfide (Li2S).
An electrolyte disposed between a cathode and an anode acts as a medium through which lithium ions move.
On the other hand, during charging, sulfur at a cathode is oxidized by releasing electrons and lithium ions at an anode are converted into lithium metal by reduction reaction by receiving the electrons.
During charging, reaction in which sulfur releases electrons to form an S—S bond occurs and reaction in which lithium ions are reduced into lithium metal by receiving electrons at a Li anode surface occurs.
When bonds in cyclic sulfur are broken by reduction reaction, a polysulfide ion of Sn2− where n is the length of sulfur chain is formed. In this regard, n may be 8 or more, and cyclic sulfur and the polysulfide ion may be converted into a polysulfide ion having a relatively long chain through the following reaction:S8+Sn2−→Sn+82−.
The most notable characteristics of such lithium-sulfur battery systems are far higher theoretical energy density than that of other battery systems. High energy density is derived from high specific capacities of sulfur and lithium. However, to achieve high energy density, high utilization of sulfur and lithium needs to be secured.
In addition, lithium-sulfur batteries may be manufactured at reduced manufacturing cost.
In particular, sulfur, which is inexpensive and abundant, is used as a cathode active material and, accordingly, manufacturing costs of a lithium-sulfur battery may be reduced. In addition, while anode fabrication of a lithium ion battery includes preparation and coating of a slurry, drying, and pressing, a lithium-sulfur battery uses lithium metal as an anode without pre-treatment thereof and thus manufacturing processes may be simplified and, consequently, manufacturing costs may be reduced. In addition, while in lithium ion batteries gases generated when lithium is intercalated into a carbon material as an anode active material are removed through activation, in a lithium-sulfur battery, gases are not generated because lithium ions are deposited on a surface of lithium metal and thus an activation process is not needed. Due to omitting of such process, there are great effects in reducing manufacturing costs of batteries.